Circuits for generating a clock signal are often required in modern electronic systems, such as computer systems, communication systems, and video systems. A phase locked loop is often selected to provide a clock signal in such systems. A phase locked loop usually includes a voltage-controlled oscillator, a phase comparator, and a reference frequency source. Unfortunately, a phase locked loop has several disadvantages when used to generate a clock signal in modern electronic systems. A phase locked loop often requires an extra pin for receiving a reference signal from the reference frequency source. The voltage controlled oscillator and the phase comparator require a large amount of space on a die. And a reference signal from the reference frequency source may not be available when the electronic system is operating in a hibernate or other power conservation mode.